On 11/09/2011 05:31 AM, Andrea Arcangeli wrote: > On Wed, Nov 09, 2011 at 01:31:07AM +0530, Srivatsa S. Bhat wrote: >> On 11/08/2011 08:59 PM, Andrea Arcangeli wrote: >>> Lack of set_freezable_with_signal() prevented khugepaged to be waken >>> up (and prevented to sleep again) across the >>> schedule_timeout_interruptible() calls after freezing() becomes >>> true. The tight loop in khugepaged_alloc_hugepage() also missed one >>> try_to_freeze() call in case alloc_hugepage() would repeatedly fail in >>> turn preventing the loop to break and to reach the try_to_freeze() in >>> the khugepaged main loop. >>> >>> khugepaged would still freeze just fine by trying again the next >>> minute but it's better if it freezes immediately. >>> >>> Reported-by: Jiri Slaby <jslaby@xxxxxxx> >>> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> >>> --- >>> mm/huge_memory.c | 3 ++- >>> 1 files changed, 2 insertions(+), 1 deletions(-) >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 4298aba..67311d1 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -2277,6 +2277,7 @@ static struct page *khugepaged_alloc_hugepage(void) >>> if (!hpage) { >>> count_vm_event(THP_COLLAPSE_ALLOC_FAILED); >>> khugepaged_alloc_sleep(); >>> + try_to_freeze(); >>> } else >>> count_vm_event(THP_COLLAPSE_ALLOC); >>> } while (unlikely(!hpage) && >>> @@ -2331,7 +2332,7 @@ static int khugepaged(void *none) >>> { >>> struct mm_slot *mm_slot; >>> >>> - set_freezable(); >>> + set_freezable_with_signal(); >>> set_user_nice(current, 19); >>> >>> /* serialize with start_khugepaged() */ >>> >> >> Why do we need to use both set_freezable_with_signal() and an additional >> try_to_freeze()? Won't just using either one of them be good enough? >> Or am I missing something here? > > set_freezable_with_signal() makes khugepaged quit and not re-enter the > sleep, try_to_freeze is needed to get the task from freezing to > frozen, otherwise it'll loop without getting frozen. > Sorry, I still don't get it. Correct me if I am wrong, but my understanding is this: There are 2 ways to freeze a freezable kernel thread (one which has unset the PF_NOFREEZE flag by calling set_freezable()): set TIF_FREEZE flag and, a) send a signal if PF_FREEZER_NOSIG is unset for that kernel thread (due to the call to set_freezable_with_signal()). Then, try_to_freeze() will get called in the signal handler. b) otherwise, just wake up the kernel thread and hope that the kernel thread itself will call try_to_freeze() sometime soon. Now coming to your patch, Case 1: You use set_freezable_with_signal() instead of set_freezable(): In this case, since the kernel thread doesn't block signals for freezing, it will get a signal (with TIF_FREEZE set) and the signal handler will call try_to_freeze(). So, no need for additional try_to_freeze() here. Case 2: You add the extra try_to_freeze(): In this case, the freezer will wake up the kernel thread, which in turn will now execute the newly added try_to_freeze() and will get frozen successfully. So, no need for set_freezable_with_signal() here. Rafael, am I right? Thanks, Srivatsa S. Bhat -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>